Running 26.2 miles can push any athlete to their physical limits. Travelling the distance without the presence of gravity makes that workout harder.

This weekend, British astronaut Tim Peake will becomes the first man to run a marathon in space.

Peake, who is four months into his six month stint aboard the International Space Station, is set to, virtually, tackle the London Marathon course along with 38,000 runners on April 24. Unlike those with their feet firmly on the ground, Peake will be strapped to the station's custom-built treadmill.

"He has to wear a harness system, which looks a bit like a rucksack. He has two bungee cords attached to the left and right of it, that pull him back to the surface of the treadmill," Peake's personal trainer Patrick Jaekel tells WIRED.

The weightless astronaut, who will have an empty path in front of him, won't be at an advantage though. The harness' straps pull him back to the treadmill and mimic his bodyweight – 60-80 per cent of his weight will be put back onto his body. "That means having to run a marathon with your bodyweight lying on your shoulders," Jaekel explains. "It's very tough and very hard." Peake, like many other runners, has also experienced chafing – but from the harness rather than his clothes.

One of the most pressing concerns on any marathon runner's mind will be whether they've done enough training. Jaekel says that the Brit went into space fit enough to run the marathon, but the majority of his exercise in space has been to prevent his body deconditioning to a level where it wouldn't be safe to run.

Peake won't have done as much as many others taking part in the marathon, but his space-based exercise routine is still hugely demanding. Each astronaut exercises for around two and half hours a day; the routines are built into their packed, science experiment-filled time on the ISS. If they don't exercise using the onboard treadmill, exercise-bike equivalent and other resistance machines, their bodies weaken dramatically in a weightless environment.

"Most astronauts come back with less muscle, less bone, and with a less efficient and a lower capacity for exercise," David Green, a lecturer of Human and Aerospace Physiology at University College London tells WIRED. No astronaut will go to space and not exercise, he adds, as the body's response to the absence of gravity is to simple use less resources.

"While astronauts normally do lots of exercise they certainly don't normally do distances that are equivalent to the marathon," says Green. For Peake – who said "I don't think you can ever do enough training" in a video briefing on Wednesday – preparation has involved runs longer than those of his fellow astronauts. These have typically last from anywhere between 40-60 minutes, with a smattering of longer runs of taking place at the weekends. In March Peake completed a 10 mile run at 7.5mph.

"We've focussed very intensely on the postural exercises, as well as the shoulder exercises," says Jaekel. The concentrated effort on the astronaut's shoulders will combat the extra load placed on his body by the harness.

Peake won't be the first astronaut to take on a marathon while orbiting around the world. In 2007 Nasa astronaut Sunita Williams, who was 41 at the time, completed the Boston Marathon in approximately four hours and 24 minutes. It took her 90 minutes of running before the orbiting capsule, travelling at 28,000kmh, made one trip around the world.

"He can't officially say it but he wants to beat her time," jokes Green. "In doing so he will create the record for a male astronaut doing the marathon." Whether or not Peake can go faster than Williams, it's unlikely he'll beat his own best time for the London race – three hours, 18 minutes and 50 seconds, set in 1999. Peake has previously said he's targeting a time between three and a half and four hours for his first space marathon.

Since Williams's effort, the running facilities on the space station have been upgraded, with a T2 Colbert Treadmill being installed in the station's Node-2 in 2009. The European Space Agency (ESA) says it has been "designed so as not to shake the rest of the Station". Despite the upgraded facilities Peake will still be confined to the small module with no windows or support from the London crowds.

He will, however, experience the London 'sights' from an app on his iPad. The RunSocial app will track Peake's pace as he takes each stride on the treadmill and move the graphic simulations around the course. As well as providing a distraction, Jaekel explains the app is crucial for "psychological support" and is better than "having him stare against a white wall for four hours".

The trainer, who will also be watching and monitoring his progress from the ESA astronaut centre near Cologne in Germany, hopes that the BBC's live feed will also be beamed to space for Peake to watch.

Those completing the race on Earth will celebrate completing the race with deserved rest, and maybe an ice bath. But for Peake, the exercise will restart the next day. "From the next day onwards we have to go into the nominal exercise programme again," says Jerkel.

"We can't afford to take him off exercise for three or four weeks to recover. We need to prepare him to come back to Earth again."